In many wireless communication systems, a plurality of user equipments share resources on a common medium for transmission. One option of avoiding resource conflicts is that for example a base station performs a scheduling or allocation of resources to selected user equipments. Dynamic allocation of the resources during operation of the communication network can significantly increase the transmission efficiency so that the resources are not left unused if some of the user equipments have presently no or only a small amount of data to transmit while others require more resources.
Resources may be subdivided into resource blocks which can be allocated individually or in groups. Depending on the transmission technology, a resource block can for example be defined by a frequency range and a time interval in which a device is allowed to transmit data.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), the uplink Resource Blocks (RB) are dedicated to users by means of uplink scheduling grants (SG) being transmitted on the Physical Downlink Control Channel (PDCCH). The uplink grants are addressed to the Cell-Radio Network Temporary Identifier (C-RNTI) of the user equipments.
To initiate a transmission, a user equipment requests uplink resources by transmitting a Scheduling Request. In LTE this can be done using the Physical Uplink Control Channel (PUCCH). The radio network, e.g. the Evolved Node B (eNB) controlling the cell where the user equipment is located, selects the resource blocks to be allocated to the user and can select also the uplink transport format, defining parameters associated with the uplink transmission, like e.g. transport block size, physical layer coding, and modulation.
However, the signalling of a scheduling request and scheduling grant before a transmission gives rise to latency and signalling overhead in the communication system and thus reduce the transmission efficiency.
A contention based (CB) transmission scheme has been proposed to improve, i.e. reduce, the latency of the system. The contention based scheme basically comprises broadcasting an uplink grant in the cell to which any uplink time aligned User Equipment (UE), that has data to send may opportunistically respond with a data transmission. For identifying such contention based grants an identifier, currently referred to as Contention Based-Radio Network Temporary Identifier (CB-RNTI), is used on the Physical Downlink Control Channel PDCCH. The available CB-RNTIs in a cell can be either broadcasted in the cell, e.g. to be used for initial access, signaled to each user using dedicated Radio Resource Control (RRC) signaling, e.g. during RRC connection setup, or signalled using an RRC reconfiguration message. The CB-RNTIs may also be specified, e.g. hard-coded in the standard.
In order to improve performance of transmission in the downlink as well as the uplink direction, LTE in non-contention based mode, i.e. contention free mode, uses Hybrid Automatic Repeat Request (HARQ). The basic idea of HARQ, for uplink transmission, is that the eNB, also referred to as the base station, after receiving data in an uplink subframe attempts to decode it and then reports to the user equipment that the decoding was successful by sending an acknowledgement (ACK) or that is was unsuccessful by sending a negative acknowledgement (NACK) on a Physical Hybrid-Automatic Repeat Request (ARQ) Channel, often referred to as PHICH. In the latter case of an unsuccessful decoding attempt, the user equipment thus receives a NACK in a later downlink subframe, and can retransmit the data that was not successfully received at the eNB.
However, in the case of an error in the decoding of a transmission from a user equipment on a contention based grant, the eNB will not be able to determine the identity of the transmitting user equipment, and will therefore not be able to correctly combine retransmissions with transmissions from different user equipments transmitting on the same contention based grant. Thus, according to the concept of contention based transmission, a single grant can be used to signal several contention based allocations. As a consequence, there might be several successful and unsuccessful transmissions based on one single grant. Therefore, one bit ACK/NACK gives very little information that might be wrong feedback for some or most of the users.